We propose to investigate the structural basis of the interaction of LCAT with HDL. We will complement these in vitro studies with analyses of HDLs of subjects with LCAT deficiency. Because these patients have very low HDL-C levels but apparently little increased cardiovascular risk, their HDL might provide important insights about HDL functions that are cardioprotective but do not involve HDL-C. We have three specific aims. First, based on the crystal structures of apoA-I and apoA-IV, we will introduce specific point mutations into apoA-I to determine their impact on its binding sites and activation of cholesteryl ester synthesis by LCAT. In parallel studies, we will use chemical cross-linking together with high-resolution mass spectrometry to identify the precise sites of interaction of LCAT with apoA-I in reconstituted HDLs. Using these data, we will test structural models of HDL established with reconstituted and native HDLs by Segrest and Davidson. Second, we use a method we recently developed, termed calibrated ion mobility analysis, to quantify the concentration and size of HDL species in the blood of control and LCAT deficient subjects. We hypothesize that normal or higher levels of certain HDL subspecies are sufficient for cardioprotection in LCAT-deficient humans, even in the absence of other forms of HDL. Third, using macrophages, we will assess the sterol efflux capacity of HDLs from LCAT-deficient and control subjects. Efflux by ABCA1 and ABCG1 will also be quantified, using genetically engineered cell lines. We will use both serum HDL (serum depleted of apoB-containing lipoproteins), isolated HDL, and reconstituted discoidal and spherical HDL particles for our studies We hypothesize that the very small HDL subspecies present at normal levels in LCAT deficient subjects is a potent mediator of sterol efflux and HDL-mediated cardioprotection.